1. Field of the Invention
The present invention relates generally to thermal printers and, more particularly, to thermal printers which compensate for variations in power supplied to a multiple heating element thermal print head.
2. Description of the Related Art
As is well known in the art, a thermal print head utilizes a row of closely spaced resistive heat generating elements, known as thermal print elements, which are selectively energized to record data in hard copy form. The data may comprise stored digital information relating to text, bar codes or graphic images. In operation, the thermal print elements receive energy from a power supply through driver circuits in response to the stored digital information. The heat from each energized element can be applied directly to thermal sensitive material or can be applied to a dye-coated web to cause diffusion transfer of the dye to paper or other receiver material. The Kodak XL7700 digital continuous tone printer contains such thermal print elements and operates in this fashion.
The transfer of dye from the web to a picture element, known as a pixel, on the receiver material is a function of the power dissipated in the associated resistive heat generating element. The power dissipated in a thermal print element is equal to the square of the voltage drop across the thermal print element divided by the resistance of the element.
A typical single density image printer is shown functionally in FIG. 1. In the printing mode, an electrical voltage from the power supply, Vs, is applied across the thermal print elements Rel-Ren. The electronic circuitry that permits current to pass through one or more of the elements in a given time interval exists in the printer and is necessary to perform the printing function. For the purpose of this description, the circuitry can be simplified to a shift register SRl-Rn, enable signal El, logical gates `ANDl`-`ANDn`, and transistor switches Tl-Tn. The complexity of this electronic circuitry varies for different printers; however, each printer has the same functionality for heating of the resistive elements.
In the printing mode, the shift register SRl-SRn is loaded with a logical "1" at each location corresponding to a pixel where there is a desire to form an optical density, ie. a transfer of dye material. The outputs of the shift register SRl-SRn are logically `AND`ed with an enable pulse E1 in the logic gates `ANDI`-`ANDn.` The enable pulse E1 is formed to represent the duration that a current is desired to pass through the thermal print elements Rel-Ren. The output of the logic gates `ANDI`-`ANDn` biases transistor switches Tl-Tn to allow current to pass through the corresponding thermal print elements Rel-Ren to ground. The energy transferred to the media to form an optical density is typically a function of the voltage drop across the thermal print element and the duration of either a constant current or a pulse count that is allowed to pass through the thermal print element. In other words, the heat generated by a thermal print element can be varied by controlling the pulse width of the current to that thermal print element or by controlling the pulse count to that thermal print element. Pulse width variation provides greater resolution than pulse count variation, but pulse width variation requires more complex algorithms than pulse count variation.
The relationship of the optical density formed at a pixel to the energy dissipated in the associated thermal print element is calibrated and is expected to remain constant during the time interval between calibrations. However, the voltage applied to the thermal print element varies with the total current drawn in the printer circuit. If the voltage applied to the thermal print element is changed by, for instance, imperfections in the power supply, switches, or distribution system, or by difficult to compute resistances in the printer circuit, the relationship between the optical density formed at a pixel to the power dissipated in the associated thermal print element is also modified. These imperfections of the circuit cause a variable parasitic resistance which creates parasitic voltage drops that are related to the number of print elements activated for a print line, thereby unpredictably altering the power delivered to the print element. This power alteration results in an unpredictable or undesirable change in the optical density formed at the pixel. This change may be evident as either an increase or decrease in the optical density of the pixel.
Numerous attempts have been made to correct automatically for resistance variations, which vary over time, between thermal print elements and parasitic resistance drops in the power distribution bus inside the thermal head. Most thermal printers incorporate driver and other circuitry that control print operations so that obtaining access to the contacts of individual print head resistive heating elements is difficult. Alternatively, determining the voltage at the terminals of the print head connectors is relatively easy. However, as described, the voltage across the print head includes parasitic drops across power supply lines, interconnections, and other wiring internal to the print head. As further described, these parasitic voltage drops are related to the number of thermal print elements activated for a print line. As a result, the parasitic voltage drops vary considerably as the number of selected heating elements changes. The varying thermal print element voltage produces noticeable variations in density of the imprinted picture elements.
U.S. Pat. No 5,053,790, issued in the name of Stephenson, assigned to the assignee of the present invention, and which is hereby incorporated herein by reference in its entirely, addresses these problems and the relevant art and proposes solutions which involve the maintenance of a substantially constant voltage across the selected resistive heat elements, independent of the number of selected heat elements in any given printing line. Several other techniques have been proposed to prevent these variations and the consequent variation of the density of their resultant print. These techniques include employing separate power sources for each of the heating elements forming a thermal print head, providing an individual balancing resistor for each of the heating elements in the head, and adjusting the electrical power applied to each of the resistive elements following production of an unacceptable print. U.S. Pat. No. 4,540,991, issued in the name of Kariya, briefly identifies these relevant art approaches and sets forth a further proposal to employ a resistance value variation detector selectively connected to each of the resistive elements in order to derive compensation data based upon resistance variation in the elements. The actual resistance values are retained in a memory at addresses corresponding to each of the resistive elements in the print head and each value is multiplied by a compensation signal to compensate thereby the printing data for each element before that data is applied to the shift register stages of the thermal print head. A similar technique is disclosed in U.S. Pat. No. 4,887,092, issued in the name of Pekruhn, and U.S. Pat. No. 4,996,487, issued in the name of McSparran, where the resistance check values are employed diagnostically or employed to indicate the temperature of the resistive element between each printing line.
Moreover, U.S. Pat. No. 4,786,917, issued in the name of Hauschild, teaches a simple but effective signal processing improvement for a thermal printer which provides enhanced continuous tone dye density images. However, none of the aforementioned patents addresses the problem of correcting for power supply loading caused by parasitic voltage drops. These voltage drops are related to the number of print elements turned on for a print line. The parasitic voltage drops vary the power delivered to each print element, thereby producing noticeable variations in density of the imprinted picture elements or pixels. When more than one heating element is activated, the loading of the electrical circuit varies with the number of elements activated. This loading variation causes the power that the individual heating element receives to vary, which in turn causes the density of the printed pixel to vary from the desired value. Since this loading variation results from a number of different factors, the precise variation during any activation of more than one heating element can be difficult to compute. For instance, the resistance of one heating element can vary slightly from that of another heating element. Further, because resistance changes with temperature, the multiplicity of connections between all of the heating elements add a further specific resistance that causes the power supply voltage to vary. As previously mentioned, if the voltage applied to the thermal print element is changed by some mechanism, such as these difficult to compute resistances, the relationship between the optical density formed at a pixel to the power dissipated in the associated thermal print element is also modified. The result of this change is that the optical density formed at the pixel is not the desired optical density. This change may be evident as either an increase or decrease in the optical density of the pixel.
In addition, U.S. Pat. No. 5,109,235, issued in the name of Sasaki, teaches a recording density correcting apparatus in a recorder for performing a recording operation at a multiple gradation by a thermal head having a plurality of heating resistors. Sasaki determines how many pulses go to each heating element at the start, and then constructs a histogram to adjust the number of supplied pulses depending on the voltage shown by the histogram. However, Sasaki does not address the problem of adjusting for only a portion of heating elements instead of all of them at once.
Consequently, a need has been felt for providing an apparatus and method which overcomes the variations in image density from the desired density due to the unpredictable parasitic resistance encountered from a portion of a plurality of heat generating elements activated in a thermal print head.